Fellowes appeared on This Morning to promote his new period drama Doctor Thorne, which stars Tom Hollander, Rebecca Front and Ian McShane.

Host Phillip Schofield asked whether he had managed to land such a stellar cast because they were aware of what a hit Downton Abbey became.

But the writer said he has no idea what made the ITV drama, which ended last year, so popular.

(Picture: Ken McKay/ITV/REX/Shutterstock)

He said: ‘You must never think like that you know. If you’re lucky enough to have a big hit you don’t ever really know why. When people say, “Oh I think it’s because of this”, they don’t really know why.

TDownton Abbey ended after six series last year (Picture: ITV)

‘Nobody knows why. This show goes through the roof, while this show – which is very, very good – nobody cares about.

‘There’s no science to it. It’s just luck. One should never expect luck. One hopes for luck but never expect it.’